Optical spectroscopy and photometry of SAX J1808.4-3658 in outburst

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Elebert, Patrick
Reynolds, M. T.
Callanan, Paul J.
Hurley, Daniel Jason
Ramsay, G.
Lewis, F.
Russell, D. M.
Nord, B.
Kane, S. R.
DePoy, D. L.
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We present phase resolved optical spectroscopy and photometry of V4580 Sagittarii, the optical counterpart to the accretion powered millisecond pulsar SAX J1808.4-3658, obtained during the 2008 September/October outburst. Doppler tomography of the NIII lambda 4640.64 Bowen blend emission line reveals a focused spot of emission at a location consistent with the secondary star. The velocity of this emission occurs at 324 +/- 15 km s(-1); applying a 'K-correction', we find the velocity of the secondary star projected on to the line of sight to be 370 +/- 40 km s-1. Based on existing pulse timing measurements, this constrains the mass ratio of the system to be 0.044(-0.004)(+0.005), and the mass function for the pulsar to be 0.44(-0.13)(+0.16)M(circle dot). Combining this mass function with various inclination estimates from other authors, we find no evidence to suggest that the neutron star in SAX J1808.4-3658 is more massive than the canonical value of 1.4M(circle dot). Our optical light curves exhibit a possible superhump modulation, expected for a system with such a low mass ratio. The equivalent width of the CaII H and K interstellar absorption lines suggest that the distance to the source is similar to 2.5 kpc. This is consistent with previous distance estimates based on type-IX-ray bursts which assume cosmic abundances of hydrogen, but lower than more recent estimates which assume helium-rich bursts.
Accretion , Accretion discs , Binaries , Close , Stars , Individual , V4580 Sagittarii , Stars , Neutron , Pulsars , Individual , SAX J1808.4-3658 , X-rays , Binaries
Elebert, P., Reynolds, M. T., Callanan, P. J., Hurley, D. J., Ramsay, G., Lewis, F., Russell, D. M., Nord, B., Kane, S. R., DePoy, D. L. and Hakala, P. (2009) 'Optical spectroscopy and photometry of SAX J1808.4−3658 in outburst', Monthly Notices of the Royal Astronomical Society, 395(2), pp. 884-894. doi: 10.1111/j.1365-2966.2009.14562.x
© 2009, the Authors. Journal compilation © 2009, RAS